The invention is directed to a filter circuit. In particular, the invention relates to low-pass filter circuits for applications in integrated circuit technology.
Filter circuits of a higher order are often required in conjunction with integrated circuits, as known, for example, from U. Tietze, Ch. Schenk, Electronic Circuits--Designs and Applications, Berlin, Heidelberg, 1991, pages 376-379. Such filter circuits cascade a number of sub-filters of the first or second order. The sub-filters are designed as individual active filters. The filters should conform to strict tolerance demands. It is difficult to achieve the strict tolerance levels in cascaded filters which have been combined using standard integration techniques. In addition, design problems occur in filter circuit applications for low frequencies because a significant portion of the chip area is used to incorporate the high capacitances and resistances which are needed.
A further disadvantage of these filter circuits occurs when high-impedance resistors are used. In addition to strict tolerance demands, bias currents are always present and may affect the circuit arrangement even more significantly than undesired offset voltages. Due to the need for precise filters with large time constants, filter circuit design technology has had to resort to external resistors and capacitors in conjunction with integrated amplifiers. Difficulty in these filter circuit designs occurs, especially in applications involving filters of a higher order where numerous terminal pins are required. Also, problems may occur because the offset voltages from the individual amplifiers correspondingly increase.